1. Technical Field of the Invention
The present invention relates, in general, to the mobile communications field and, particularly, to a system and method for enhanced time of arrival measurements for mobile station positioning utilizing geographical characteristics of the mobile communications network.
2. Background and Objects of the Invention
Mobile radio position determination is an emerging field that has gained a great deal of interest lately, and it is now desirable to include a position determination capability in future cellular mobile radio systems. The Time Difference of Arrival (TDOA) position determination method, which is known from military applications, has been used successfully for determining the position of mobile radio terminals. A typical TDOA position determination system can be either terminal based, whereby Time Of Arrival (TOA) measurements are made on the "downlink" in the mobile station (MS), or network based, whereby the network performs the TOA measurements on the "uplink" in the radio base transceiver stations (BTSs). These TOA measurements are then used to calculate TDOA parameters and estimate the position of the mobile station (MS).
A network-based method for determining the position of cellular mobile stations is disclosed in commonly-assigned Swedish Patent Application No. 9303561-3 to R. Bodin. In order to determine the position of a mobile station, a handover procedure is initiated between a serving base transceiver station and the new base transceiver station. The mobile station transmits access request signals to a new base transceiver station. The base transceiver station measures the time delay for the access request signal to travel between the mobile station and the base transceiver station. This procedure is repeated between the mobile station and one or more additional base transceiver stations. A service node in the cellular network calculates the position of the mobile station by utilizing information about the known positions of the base transceiver stations and the measured access time delays.
This network-based method of determining the position of cellular mobile stations relies on so-called asynchronous handovers, where the target base transceiver stations measure the access delays to the mobile station. Each access delay is used as a measure of the distance between the mobile station and the respective base transceiver station. In specific situations, i.e., MS positioning, a number of BSs may concurrently tune to the same transmitting MS in order to make time delay of arrival measurements between the MS and the BSs. The time delay of the signal transmitted by the mobile station during a handover procedure may be measured by the BSs for use by a processing unit to determine the position of the mobile station. The mobile station transmits signals during the handover operation as a series of bursts.
Although the above-described document illustrates considerable progress in the cellular position determination field, there are still a number of deficiencies to be improved upon. For instance, in existing GSM systems having TOA-based mobile station positioning capabilities, the determination of a mobile station position is not highly accurate. The calculation of the number of access bursts for reception from a mobile station is based upon the type of application and/or the degree of accuracy specified. In addition, the identification of particular time measuring units for use in measuring the time of arrival of transmitted access burst signals is based upon only a general location of the mobile station, such as information gleaned from cell identification and timing advance (TA) parameters. As a result, mobile network resources may not be optimally utilized in determining mobile station position. Network resources in existing GSM systems may be overly utilized to determine mobile station position, thereby reducing the quality of other services provided by the telecommunications system. Alternatively, network resources may be under utilized, resulting in a less accurate mobile station position determination. Consequently, there exists a need for an improved TOA based, mobile station positioning system.
It is an object of the present invention to provide a TOA based positioning system which efficiently and accurately determines the position of a mobile station in a mobile telecommunications network.
It is another object of the present invention to substantially optimally balance accurately determining the position of a mobile station within a mobile telecommunications network against providing wireless speech communication therein.
It is yet another object of the present invention to provide such a system wherein the method of determining the location of a mobile station is based in part upon geographical characteristics of the mobile telecommunications network.
Another object of the present invention is to provide such a system wherein the method determining the location of the mobile station is based upon the location of time measuring units within the mobile telecommunications network.